Portugal, S.J., Hubel, T.Y., Fritz, J., Heese, S., Trobe, D., Voelkl, B., Hailes, S., Wilson, A.M. & Usherwood, J.R. (2014). Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight. Nature 505, 399-402. doi:10.1038/nature12939
See also the Structure & Motion lab webpage for this Ibis project.
Heart rate monitoring of pelicans suggest flying towards the back of a V is metabolically less demanding .
Theoretically, and demonstrated with fighter aircraft, there could be an aerodynamic energetic benefit to flying in a V (but takes a lot of control).
Pigeons flap at a higher rate in – and especially towards the back of – a flock, probably working harder.
There are many reasons a bird at the back of a V might have a lower heart rate – more relaxed etc.
There are many reasons for flying in a V – military aircraft do it while avoiding aerodynamic interactions so as to keep good sky view and not crash into each other
Flapping wings make a complicated wake, so possibly making use of aerodynamic advantage difficult.
That – on average for ibises flying in a V – birds positioned themselves almost exactly in the span wise (i.e. to left or right) position predicted to give most benefit from simple fixed-wing aeroplane theory.
That birds flying in this span wise position time their wing beats, depending on their position streamwise (i.e. to the back), to match wingtip paths, and so the oscillating regions of upwash.
That birds that find themselves directly behind another bird show the opposite wingbeat phasing, thus putting their wings in regions as much away from the oscillating regions of downwash (bad) as possible.
We conclude that:
Birds flying in a V are doing so for aerodynamic reasons.
And they are better than we thought at dealing with complex wakes.
We don’t know:
Whether they do this with a ‘rule of thumb’ or by sensing the air (or both). To be investigated.